It is possible to cause the Google toolbar to show an incorrect number. However, it is not very easy to do. Not that I'll be listing the steps or anything
The number that you see in the toolbar is calculated based on the number of inbound links, and how many links each of those sites has, going back at least three iterations. As such, to calculate page rank you would need to know the toolbar pagerank of every site linking to yours, and be able to find every site linking to yours. When you look at the links to a site through a search engine operator, however, the listing is only a random sampling of the links to that site, generally what the search engine calculates as being the most relevant inbound links at that moment to that particular site. It does not give a very reliable idea of how many links the site truly has, and it does not show the highest ranking (or necessarily the lowest either) links.